Hmmm.....none of my top 5 start with A

Anyway:
(Nice Dream) - Radiohead
^M1-1=-aED1[O][EF1[n-1]+Fext1[n-1]] - Aphex Twin
1234 - Ozomatli
137 (Rinse) - Squarepusher
1855BC - Meat Beat Manifesto & The Orb
The first track beginning with 'a' is at no. 25